The great majority of acrylic acid produced commercially is prepared via the catalytic, two-stage vapor phase oxidation of propylene. In the first stage propylene is oxidized with air to acrolein and then fed directly to the second stage where the acrolein is further oxidized with air to acrylic acid. The catalysts used in the two stages are mixed metal oxides that have been optimized for their respective chemistries. The first stage catalyst is composed of mainly molybdenum and bismuth oxides with several other metals. The second stage catalyst is also a complex mixed metal oxide catalyst where the oxides employed are primarily of molybdenum and vanadium. Several other components have been incorporated in the catalyst to optimize activity and selectivity. Acrylic acid yields of 80-90% from propylene have been realized for these commercial catalyst systems.
Downstream of the oxidation reactors, additional processing steps are utilized to cool, collect, and purify acrylic acid present in the reactor exit gas stream. The original acrylic acid processes used water as the diluent, which meant that the reactor product yielded an approximately 35% aqueous acrylic acid solution upon quenching and separation of the non-condensable gases in the quench or absorber tower. This low concentration of acrylic acid in water had to be recovered via a solvent based extraction followed by several distillation steps to generate a technical grade acrylic acid. Technical grade acrylic acid is used to prepare the higher purity glacial acrylic acid or to prepare acrylates, i.e., esters of acrylic acid. When recycle gas technology was introduced, the aqueous acrylic acid obtained in the quench tower was concentrated to approximately 65% which allowed the use of solvent-based azeotropic distillation to remove the water. The crude acrylic acid after water removal was then subjected to several distillation steps to yield a technical grade acrylic acid. An alternate technology for recovery of the 65% aqueous acrylic acid involves the introduction of a high boiling solvent in the quench tower to absorb the acrylic acid via a solvent swap. The base of the quench tower yields acrylic acid dissolved in this high boiling solvent instead of water. The acrylic acid is then subjected to further distillation steps for recovery from the high boiling solvent to yield technical grade acrylic acid.
U.S. Pat. No. 6,596,129 teaches using two condensers in series in a (meth)acrylic acid distillation process.
U.S. Pat. No. 6,878,239 teaches the addition of an inhibitor to a steam jet ejector condensate, surface condensers, and a liquid ring (“nash”) vacuum pump to prevent polymer formation. The vacuum section is a separate system from the overhead condensers.
U.S. Pat. No. 5,980,698 discloses a distillation column to produce vacuum gas oil in a separation process using a cooled product stream entering the separator.
U.S. Pat. No. 6,019,820 teaches the use of a liquid jet eductor as a gas compression system supplied with high pressure liquid to increase the gas outlet pressure.
Conventional processes for producing acrylic acid generally add solvents in the distillation column or in the condensers. Additionally, water in the form of steam is often introduced through the use of steam jet ejectors, which help reduce the operating pressure within distillation columns. For example, U.S. Pat. No. 7,288,169 teaches the use of steam jet ejector-based vacuum systems for acrylic acid distillation columns. The systems disclosed by U.S. Pat. No. 7,288,169 comprise vertical shell-and-tube surface condensers, steam jet ejectors, and a steam jet condensate collection tank, which is used to collect and recycle the steam jet condensate.
U.S. Pat. No. 6,677,482 teaches an improvement to the use of steam jet ejectors and surface condensers by recycling the condensed steam back into the separations process.
A schematic process flow sheet of a prior art configuration in which shell-and-tube condensers and steam jet ejectors condense the overhead product of a finishing column is shown in FIG. 2. In FIG. 2, finishing column overhead stream 8 is removed from finishing column 17 as a vapor and is passed through at least one vertical shell-and-tube surface condenser 19 to form a finishing column overhead condensate stream, at least a portion of which is passed, via line 4, to dehydration column heater/reboiler 12.
Reduced operating pressure within finishing column 17 is maintained in this prior art process through the use of a common multistage steam jet ejector vacuum system 50. Such a vacuum system is well known in the art and comprises a plurality of steam jet ejectors and associated interstage jet condensers. As illustrated in FIG. 2, steam is supplied to a first jet ejector 51, which draws organic vapors and non-condensable gases out of condenser 19 via line 25. The organic vapors and the steam are ejected into shell-and-tube surface condenser 52, where they are at least partially condensed, forming a first aqueous steam condensate stream 54, which further comprises condensed organics. Steam is also supplied to a second jet ejector 55, which draws residual organic vapors and non-condensable gases out of condenser 52 via line 53. The residual organic vapors and the steam are ejected into shell-and-tube surface condenser 56, where they are at least partially condensed, forming a second aqueous steam condensate stream 58, which further comprises condensed organics. Non-condensable gases and any remaining uncondensed organic vapors are vented from the vacuum system via vent line 57. Steam condensate streams 54 and 58 are collected in condensate receiver 59. Because the steam condensate in receiver 59 comprises condensed organics, aqueous stream 60 cannot be directly discharged to the environment and therefore requires additional processing. In some instances, it may be possible to transfer aqueous stream 60 to another separations system. In other embodiments, however, such a “recycling” option is not economically justified and stream 60 is instead transferred to an aqueous wastewater treatment system or even a thermal oxidizer for disposal.
An improved process for producing acrylic acid is disclosed in U.S. Pat. No. 8,242,308, which is incorporated in its entirety herein by reference. U.S. Pat. No. 8,242,308 discloses a process which does not require the addition of an azeotropic solvent or other solvent in the distillation columns, thus eliminating the need for additional purification of the acrylic acid product stream. WO 2009/123872 describes a method for starting up a (meth)acrylic acid production process of the type disclosed in U.S. Pat. No. 8,242,308.
It would be desirable to have an acrylic acid production process which eliminates or minimizes the generation of an aqueous waste stream and/or provides reduced capital and operation costs.